[Docs] Document Crossbow builds outside of code

Find a way to document the different builds that are available on-demand and run regularly (nightly) outside of docker-compose.yml & tasks.yml.

Something like a "build matrix", ideally we would also have a "supported-config matrix" to compare against each other.

CC @raulcd  

**Reporter**: [Jacob Wujciak](https://issues.apache.org/jira/browse/ARROW-16370) / @assignUser

_{**Note**: *This issue was originally created as [ARROW-16370](https://issues.apache.org/jira/browse/ARROW-16370). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start by reviewing docker-compose.yml and tasks.yml to identify the on-demand and nightly Crossbow builds they currently describe. Trace any additional build definitions mentioned there, then document the available build matrix and, if the configuration information supports it, a comparison with the supported-configuration matrix.

Written by the indexing model from the issue text.